Most antenna developments up to this point have focused on traditional passive power transduction mechanisms, which rely on matching the radiation resistance of the antenna structure to the intrinsic impedance of free space and matching the output terminal impedance of the antenna structure to the input impedance of the receive system. There are numerous passive matching techniques and geometries that have been developed over the many years antenna technology has been in existence. Most of the innovation in antenna technology has been in the Aerospace, Defense, and Satellite Communications industries, while Commercial Radio and Television have long relied upon technology that has been available for over 40 years.
The current Television (TV) spectrum extends from 54 MHz to 806 MHz, corresponding to wavelengths ranging from 5.6 meters to 37 centimeters respectively. The more efficient, passive TV antenna designs commonly used can be relatively large and involve fairly elaborate geometries, to accommodate this range of frequencies. Typical TV antenna designs range from simple narrowband dipole structures, designed to be ½ of a wavelength at the frequency of interest, to more exotic broadband structures such as the log periodic dipole array, which consists of several dipoles of decreasing size arranged coaxially. An efficient log periodic array can exceed 3 meters in length, with the longest dipole element reaching up to 2.7 meters. An array of this size can achieve gains as high as 5 dB to 9 dB over that of a dipole, which typically is around 2 dBI at a resonant ½ wavelength. This advantage over the dipole is a result of directive gain associated with the particular combination and relative phasing of the array elements. The single dipole has a bi-directional radiation/reception pattern and a bandwidth of around 30%, whereas the log periodic array is designed for a highly directional radiation/reception pattern and can accommodate bandwidths of several octaves.
Electrically small antennas are becoming more common in recent years due to size constraints imposed on many wireless consumer electronics. Also, there is a growing interest in this technology within the TV broadcast community as applied to indoor analog and digital TV reception and the indoor reception of Datacasting services. For example, a consumer residing in an apartment may require a high-gain directive antenna to receive broadcast DTV and/or an on-demand movie service via Datacasting, but does not have the space to utilize a typical log-periodic array. In this case, only an electrically small, broadband, high-gain antenna, with some directive selectivity for interference rejection, would be practical.
There are indoor antennas available to the consumer designed with these applications in mind, but most perform at low efficiencies and utilize active electronics to amplify the low-level antenna output power. Antennas such as these are often referred to as “active antennas” or “integrated active antennas”, even though they are simply passive antennas with low-noise amplifiers (LNA) conditioning the output signal. The antenna section of these assemblies are acting as power transducers and still must be impedance matched to the LNA at all frequencies of interest to be useful. As a result, the indoor TV antenna designer must utilize broadband design techniques to achieve a broadband impedance match between the antenna output and the LNA input over several octaves of the TV frequency spectrum. If the additional requirement of directivity/spatial selectivity is imposed, the design becomes much more challenging.